A high-voltage interlock connector that is not fully latched or has backed out slightly; Corrosion or water intrusion in an interlock connector or service disconnect plug; Damaged interlock wire (open circuit) between the battery pack and the control module; Failed service disconnect switch or manual disconnect lever not fully engaged; Internal fault in the battery energy control module or interlock monitoring circuit; Aftermarket alarm or remote start installation that tapped into the interlock loop; Collision damage or pinched harness causing an open in circuit K

DTC codes / U35xx · Network communication / U3533

U3533High Voltage System Interlock Circuit K Low

Q: What does code U3533 mean?

High Voltage System Interlock Circuit K Low. The high-voltage interlock loop is a safety circuit that confirms all high-voltage connectors, covers, and service disconnects are properly seated before the battery pack energizes. Code U3533 sets when the control module monitoring circuit K detects a voltage level that is too high, indicating an open or high-resistance path rather than a short to ground. The vehicle may disable the high-voltage system entirely, preventing the engine from starting or the traction battery from engaging.

The high-voltage interlock loop is a safety circuit that confirms all high-voltage connectors, covers, and service disconnects are properly seated before the battery pack energizes. Code U3533 sets when the control module monitoring circuit K detects a voltage level that is too high, indicating an open or high-resistance path rather than a short to ground. The vehicle may disable the high-voltage system entirely, preventing the engine from starting or the traction battery from engaging.

SystemNetwork

Code typeGeneric (all makes)

SeverityHigh

Fix difficultyProfessional

AreaNetwork communication

Symptoms you may notice

Hybrid or electric vehicle fails to start or enter 'Ready' mode
Instrument cluster displays a high-voltage system warning or service vehicle soon message
Often no symptoms besides the warning light if the fault is intermittent
Traction battery contactors may not close, leaving the vehicle immobile
Remote start or pre-conditioning functions may be disabled
Other unrelated warning lights may illuminate as modules lose high-voltage communication

Common causes, most likely first

A high-voltage interlock connector that is not fully latched or has backed out slightly
Corrosion or water intrusion in an interlock connector or service disconnect plug
Damaged interlock wire (open circuit) between the battery pack and the control module
Failed service disconnect switch or manual disconnect lever not fully engaged
Internal fault in the battery energy control module or interlock monitoring circuit
Aftermarket alarm or remote start installation that tapped into the interlock loop
Collision damage or pinched harness causing an open in circuit K

How to diagnose and fix it

Inspect all high-voltage interlock connectors for proper seating, corrosion, and damaged pins, starting at the service disconnect and battery pack
Check the service disconnect plug for full engagement and verify the interlock lever is locked
Measure resistance or voltage on circuit K at the control module connector and compare to the manufacturer's specification
Perform a wiggle test on the harness while monitoring interlock status on a scan tool to locate an intermittent open
Repair or replace any damaged interlock wire or connector, then clear the code and verify the high-voltage system powers up
If the circuit is intact, test the service disconnect switch and replace if faulty
As a last step, diagnose or replace the battery energy control module if all external circuits test good

Interlock faults that mimic component failures

A partially latched high-voltage connector after a recent repair is the most common cause, so always recheck work performed before the light appeared.
The interlock loop is a low-voltage safety circuit, so a code U3533 does not automatically mean the high-voltage battery or inverter has failed.
Water intrusion from a clogged sunroof drain or trunk seal can corrode interlock pins without any visible exterior damage.
Aftermarket electronics spliced into the interlock circuit often create a high-resistance path that sets this code intermittently.
Some vehicles set U3533 when the 12-volt auxiliary battery is weak because module voltage monitoring becomes unreliable during cranking.

Can I keep driving with U3533?

Do not continue driving if the vehicle will not enter Ready mode or shows a high-voltage system fault, because the traction battery may be disabled and the vehicle can lose power unexpectedly. If the light is intermittent and the car drives normally, arrange diagnosis promptly and avoid high-voltage service work until the interlock circuit is verified.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.